High-pressure plug valves are well known in the art and used principally in the oil and gas industry. A high-pressure plug valve is described in Assignee's co-pending United States Patent Application published under publication number 2006/0027779 on Feb. 9, 2006, which is entitled High-Pressure Plug Valve with Replaceable Inserts and Method of Refurbishing Same.
FIG. 1 shows a side elevational view of a plug valve 10 of this type. The plug valve 10 includes a plug valve body 12 having a top flange 14 and a stud pad 16. As will be understood by those skilled in the art, the flange 14 and stud pad 16 could be any of a flange, stud pad or threaded union. The plug valve 10 has a stem 18 for rotating a plug (not shown) within the plug valve body 12 to control fluid flow through the valve in a manner well known in the art. The plug is retained in the plug valve body 12 by a back cap 20.
FIG. 2 is a schematic cross-sectional view of a back cap 20 in accordance with the prior art. The back cap 20 has an inner end 22 and an outer end 24. The outer end 24 includes a plurality of radial bores 26 through which a tool can be inserted to install the back cap 20 in the plug valve body 12 or remove it from the plug valve body 12. The inner end 22 includes a plug support surface 28 which supports the plug of the plug valve 10. The inner end 22 further includes a seal rim 30 having a peripheral O-ring groove 32 that receives an O-ring 34 for providing a seal above pin threads 35 that engage box threads (not shown) in the plug valve body 12 to retain the back cap 20 in the plug valve body 12. The inner end 22 further includes a seal bore 36 which receives a fluid seal for sealing around a stud end of the plug. The stud end of the plug is received in a guide bore 38 that aligns the plug within a plug cage (not shown) of the plug valve body 12. The stud end also extends through a stud bore 40 for fluid pressure balance with the stem 18 shown in FIG. 1.
A problem with the back cap 20 occurs during installation of the back cap 20 in the plug valve body 12. Because the back cap 20 is retained in the plug valve body 12 by the pin threads 35, it must be rotated during installation. When the O-ring 34 encounters a sealing surface against which it seals in an interior of the plug valve body 12, the O-ring 34 resists rotation due to frictional engagement with the sealing surface. Consequently, the O-ring 34 can be damaged, or it may roll partially out of the O-ring groove 32. If this occurs the valve will leak around the back cap, even though the leak may not develop until the valve has seen a certain amount of high-pressure service.
Even if the O-ring 34 stays in the O-ring groove 32 and is not damaged during installation, a leak around the back cap can still develop due to stress on the O-ring 34. As understood by those skilled in the art, when subjected to high fluid pressures the plug valve body 12 expands, or “grows” as it is referred to in the industry. As the plug valve body 12 expands, a gap develops between an outer peripheral surface of the seal rim 30 and the seal surface of the plug valve body 12. Even though an undercut 31 is provided in the back cap 20 to permit the seal rim 30 to expand with the plug valve body 12, experimentation as shown that under very high pressures the seal rim 30 does not expand at the same rate as the plug valve body 12. This creates a space between the seal rim 30 and the plug valve body 12 that puts stress on the O-ring 34. That stress eventually causes the O-ring 34 to fail, which results in a leak around the back cap 20, requiring maintenance of the plug valve 10.
There is therefore a need for a back cap for a plug valve that provides a long service life without maintenance. There is also need for, a plug valve with a back cap that can be installed without danger of damaging back cap seals.